Fitness for Taekwondo

I'm 15 and am about to get a blue belt in Taekwondo, but I was wondering what are the best things at the gym I can do to improve my Taekwondo? Such as, how can I get stronger without gaining alot of mass that would slow me down. Thanks

I'm pretty sure you can dig up some good info on this subject with the search function. Btw, I think this would have been a better topic for the Physical Fitness forum here. Other than that, good luck on your blue belt grading.